Electronic equipment which uses a magnetron to generate microwave energy, such as microwave ovens and industrial microwave heating devices, is widely used these days. Along with the development of such electronic equipment, the regulation for suppressing unnecessary radio waves, that is, high frequency noise, has become serve. Noise regulations established by the International Special Committee on Radio Interference (CISPR) are in practice in some countries and are under consideration in other. Therefore, it is desired that measures be taken to further reduce the noise or unnecessary radio waves produced from the magnetron and their leakage.
The conventional structure of the magnetron for microwave ovens and the noise generated therefrom will be described below. FIG. 1 shows the magnetron structure for an microwave ovens of, for example, 2.45 GHz. Reference numeral (21) denotes the magnetron oscillation main body; (22), a radiator; (23), ferrite magnets; (24), the yoke; (25), the spiral directly-heated cathode; (26), the anode vane; (27), the anode cylinder; (28), the strap ring; (29), the output section; (29a), the antenna feeder; (30), the pair of pole pieces; (31), the cathode stem assembly as an input section of the magnetron; (31a), the insulating cylinder; (36), the choke coil; (37), the through type capacitor; (37a), the cathode input terminal; and (38), the shield box. A voltage of several thousands (V) is applied between the anode vane (26) and the cathode (25). A magnetic flux parallel to the axis of the cathode, that is, the axis of the cylinder is induced in an interaction space (39) between the ferrite magnets. Oscillation is thus performed. The oscillated microwave is supplied to an external load from the output section through the antenna feeder. Noise and dominant microwave oscillation are mixed in the microwave oscillation generated from the output section. An unnecessary high frequency noise component is leaked through a filter circuit which comprises the cathode input terminal, the choke coil as part of an input lead wire connected to the cathode input terminal, and a through type capacitor. This noise component varies from several tens Hz to several GHz. The filter circuit acts to attenuate a noise component leaked to a power source transformer and commercial power lines. However, in order to prevent the leakage described above, a high quality filter circuit must be used.
The noise component leaked toward the input lines is measured by the measuring circuit of FIG. 2. As shown in FIG. 3, a spectrum in which the noise component is continuously plotted until near 1,000 MHz is detected. However, in this case, the filter circuit with the choke coil and the capacitor shown in FIG. 1 is not used. Referring to FIG. 2, reference numeral (20) denotes a magnetron similar to the magnetron shown in FIG. 1; (40), a waveguide; (41), a dummy load; (42), a cathode power source; (43), a high voltage power source; (44), cathode input lines; (45), a measuring probe such as a ferrite clamp; and (46), a spectrum analyzer.
The following assumption can be made about the causes of the continuous range of magnetron noise. FIG. 4 illustrates a model of the anode and the cathode which are coaxial to the magnetron. Assume that the cathode is defined as the negative terminal and the anode is defined as the positive terminal, and that a high voltage of several thousands (V) is applied across the magnetron to emit thermoelectrons from the cathode. As indicated by the broken curve (a), the potential curve is of a concave shape and the potential is minimized in the active space (39). Meanwhile, since a DC magnetic field of 1,000 to 2,000 gauss is applied to the active space in the direction of the cathode axis as indicated by arrow (B), electrons emitted from the cathode orbit rotate around the cathode due to the relation between the electric field and the magnetic field. Because the magnetron is a crossed-field microwave tube, it has an especially long electron path. Therefore, the electrons tend to collide against the residual gas and generate a great amount of positive ions (+), compared to other linear beam electron tubes. These positive ions stay at the minimum potential as indicated by recessed portion (m) of the curve (a). The electrons in the active space are gradually neutralized with the positive ions. As a result, a potential in the active space indicated by the recessed portion (m) is increased, as indicated by reference symbol (n). The potential becomes equal to or slightly higher than that in the cathode (25). In this condition, the potential curve for the active space (39) is changed as shown by solid curve (b). In the next step, the electron charge in the active space serves to form the recessed portion (m) of the potential curve again. The position of the recessed portion (m) is estimated to be formed at about several tens .mu.m to several hundreds .mu.m from the cathode surface. This process is repeated periodically. As a result, a complicated pulsation phenomenon occurs in a current between the cathode and the anode, that is, an anode current. This anode current becomes a low frequency component, for example, a noise of several tens Hz to several mega heltz. In the vicinity of the recessed portion (minimum potential) of the curve, a plasma which is a mixture of electrons and ions is produced, and plasma vibration occurs within the plasma. This vibration is the cause of a relatively high frequency noise of, for example, several mega heltz to several hundreds MHz. It is presumed that these low and high frequency noise components are leaked to the external circuit. Therefore, the noise components constitute a relatively low frequency of less than 1,000 MHz. These noise components are leaked mainly through the input lines, and are called line noise. Furthermore, these noise components result in a variety of radio wave interference.
In order to decrease the noise components based on this assumption of the cause of noise, the recessed portion of the curve, that is, the minimum potential in the vicinity of the cathode must be substantially eliminated. Alternatively, the positive ions floating in the active space must be immediately eliminated. The unstable variation in the recessed portion of the curve can thus be eliminated.
A magnetron with a control grid near a cathode to control output power is already known. For example, a grid which controls output power is described on page 85 with reference to FIG. 2 in "CROSSED-FIELD MICROWAVE DEVICES", Vol. 11, 1961 edited by E. OKRESS. A magnetron with a grid is also described from page 293 in "Goku-cho-tampa ji-den-kan no ken-kyu (Study on Microwave Magnetron)", 1952, Misuzu-shobo, edited by Tomonaga and Kotani. The former grid controls the output power variable and the latter grid measures noise with a triode. The latter grid shows that changes in grid voltage are substantially independent of noise. On the other hand, the former grid which controls the output power is arranged to substantially surround the cathode with respect to the anode vane in order to control the electron stream. It is installed at a great distance from the cathode. Therefore, the grid does not act to catch positive ions floating near the cathode to decrease high frequency noise.